A continuing problem with the transmission of digital information over a wireless communication system is the maintenance of relatively high quality, i.e., error-free, transmission. A quantity used in the art as an indicator of the quality of a transmitted bit stream is the bit error rate (xe2x80x9cBERxe2x80x9d) of the bit stream at the receiver. The BER is a measure of the number of bit errors encountered for a given number of bits. A typical BER for a wireless communication system is on the order of 10xe2x88x926, or 1 bit error per million bits transmitted.
The BER of a given wireless transmission path is determined by a number of factors. One of those factors relates to the effects of temperature changes on the receiver circuitry, specifically the synthesizer and/or oscillator circuits. The synthesizer circuit typically consists of a phase lock loop, including a loop filter, a phase detector, a voltage controlled oscillator circuit (xe2x80x9cVCO circuitxe2x80x9d) including an oscillator, a resonator, inductors, capacitors, means for tuning the oscillator such as a varactor, and various amplifiers. As used herein, unless otherwise indicated, the synthesizer is used to refer to the phase lock loop circuitry without the VCO circuitry. The performance of some or all of these circuits exhibit some degree of temperature sensitivity. For hand-held radio receivers that are power limited, the temperature problem is typically due to heat loss since power for heaters is limited. For temperature changes that result in gradual changes in frequency at the receiver, the phase lock loop generally operates to maintain the receiver locked on to the signal frequency. For temperature changes that result in a step event, i.e., a sudden change in an operating parameter of the synthesizer and/or oscillator circuits that results in a sudden shift in frequency or phase, the BER suddenly increases until the phase lock loop can operate to mitigate the step event. This effect is exacerbated when the receiver operates in the multiplied frequency range, i.e., where the output of a relatively low frequency oscillator circuit is multiplied to achieve a higher frequency signal for transmission. Extreme temperature stability for such multiplied frequency systems is essential for error free transmission of digitally modulated data.
Previous solutions to the problem of mitigating the effects of a step event have focused on using more expensive components, either for a higher grade oscillator that is less susceptible to temperature effects or for the phase lock loop to more quickly mitigate the temperature effects to thereby minimize the loss of data and the time the BER is high after a step event. However, the prior art solutions are expensive and do not result in an appreciable decrease in data errors after a step event.
One embodiment of the present invention avoids the problems of the prior art by focusing on the temperature stability of the resonator in a broadband, very low phase noise, temperature stable, sync loss free voltage controlled oscillator. Experiments performed by the inventors show that during temperature transition periods, electro-mechanical events occur in typical VCO circuits, specifically in the resonator, that result in a step change in phase, The electromechanical events may be due to imperfections in the inductive and capacitive elements of the resonator. For instance, the layers in multi-layer capacitors may pull apart or compress together during temperature changes resulting in a sudden change in capacitance.
Embodiments of the present invention use one or more of the following techniques to minimize the step change in phase due to temperature variations of the resonator, typically temperature decreases due to heat loss. Printed microstrip inductors may be used to minimize imperfections found in inductors containing a higher volume of material. Porcelain and/or single layer capacitors may be used to minimize multi-layer capacitor effects. The resonator may be isolated on its own printed wiring board (xe2x80x9cPWBxe2x80x9d) to obtain better thermal control. The resonator PWB may be enclosed in an insulator to minimize heat loss. The insulator may function to reflect radiated heat from the resonator PWB back to the circuitry on the resonator PWB and/or the insulator may function to minimize convective cooling due to airflow over the resonator PWB. Heaters and heat-producing circuits may also be mounted on the resonator PWB and enclosed in the insulator. An example of a heater would be a positive temperature coefficient resistive heater (xe2x80x9cPTC heaterxe2x80x9d). The PTC heater may be mounted on the side of the resonator PWB opposite the side on which the resonator is mounted. The PTC heater may be mounted over a blind via extending substantially through the substrate of the resonator PWB Typical examples of heat-producing circuits would be buffer amplifiers, varactor, and oscillator. The number of connections between the resonator PWB and other circuitry in the receiver may be minimized to limit heat loss through electrical connections.
Accordingly, it is an object of the present invention to provide a novel broadband, very low phase noise, temperature stable, sync loss free voltage controlled oscillator.
It is another object of the present invention to provide a novel broadband, very low phase noise, temperature stable, sync loss free voltage controlled oscillator for a radio receiver operating in the millimeter frequency range.
It is yet another object of the present invention to provide a novel broadband, very low phase noise, temperature stable, sync loss free voltage controlled oscillator operating in a multiplied frequency range.
It is still another object of the present invention to provide a novel VCO circuit to minimize temperature changes of the resonator.
It is a further object of the present invention to provide a novel method of temperature enhancing the phase stability of the resonator in a VCO circuit and thereby ad enhancing the phase stability of the synthesizer.
It is yet a further object of the present invention to provide a novel method of controlling the BER of a received phase modulated signal from a remote transmitter by temperature stabilizing the resonator of the receiver.
It is still a further object of the present invention to provide a novel method of controlling the BER of a received digitally modulated signal from a remote transmitter by temperature stabilizing the resonator of the receiver.
It is an additional object of the present invention to provide a novel method of reducing occurrences of step changes in phase difference between the phase of the received signal and the phase of the output signal of the oscillator of the receiver by temperature stabilizing the resonator of the receiver.
It is yet an additional object of the present invention to provide a novel method for minimizing the heat loss from a resonator by mounting the resonator on a PWB separate from the synthesizer PWB and encapsulating the resonator PWB in an insulated enclosure.
It is still an additional object of the present invention to provide a novel method for minimizing the heat loss from a resonator by mounting the resonator on a PWB separate from the synthesizer PWB, disposing a PTC heater and heat-producing circuitry on the resonator PWB, and encapsulating the resonator PWB in an insulated enclosure.